In this type of configuration, shown in FIG. 1 which is a diagram of a fuel circuit, the fuel initially contained in a tank 1 passes through a filter 2, and is then drawn into an injector pump 3 by means of a feed line 4. The outlet from the injector pump 3 is connected to injectors 5 for feeding the cylinders of the engine, and excess fuel is returned to the tank 1 by means of a return line 6.
The return line 6 is at a pressure greater than the pressure that exists in the tank 1, while the feed line 4 is at a pressure that is lower than the pressure in the tank 1.
A priming pump 7 is disposed between the feed line 4 and the return line 6 of the injector pump 3 in order to re-prime it after running out of fuel or after taking action on the feed circuit. For this purpose, the priming pump 7 has inlet and outlet endpieces fitted with moving check valve members (not shown in the figure) so that when the priming pump is operated, fuel is transferred from the feed line 4 to the return line 6 (arrow 8) in order to force-feed the injector pump 3.
In a configuration with the priming pump in parallel as shown in FIG. 1, no fuel flows through the priming pump during normal operation of the engine, and as a result fuel stagnates permanently in the priming pump.
The valve members included in the inlet and outlet endpieces of the priming pump thus remain in the closed position during normal operation of the engine, and so the valve member for the inlet endpiece is subjected to suction from the feed line and the valve member for the outlet endpiece is subjected to pressure from the return line.
Unfortunately, the valve members in the endpieces are never perfectly seated against their seats, and each of them therefore presents a residual leakage rate. Depending on which one of the valve members presents the greater leakage rate during operation of the engine, the priming pump can be subjected either to excess pressure, in which case it will become slightly inflated, or else to suction, in which case it runs the risk of collapsing. This can be particularly troublesome. Since the pump is located in an environment that is very hot, it runs the risk of becoming progressively thermoformed in the position that it occupies while the engine is running.
Being thermoformed in the inflated position has no effect on the operation of the priming pump. It will still be able to perform its function whenever necessary. However, being thermoformed in the collapsed position makes the pump unusable. The invention seeks to avoid this situation.